1. Field of the Invention
The present invention relates to an exhaust gas recirculation (hereinbelow, referred to as EGR) control system which controls recirculation of a part of an exhaust gas from an internal combustion engine into an intake pipe of the engine.
2. Discussion of Background
In order to reduce nitrogen oxide as a noxious substance included in an exhaust gas from an internal combustion engine, it is remarkably effective to lower the combustion temperature. A method to recirculate the exhaust gas into the combustion chamber has been well-known as a measure to lower the combustion temperature. In FIG. 10 is shown the EGR control system which has been disclosed in JP-A-5593950. In this Figure, reference numeral 1 designates an engine, reference numeral 2 designates an intake manifold, reference numeral 3 designates an exhaust manifold, reference numeral 4 designates a fuel-supply system which is arranged in the intake manifold 2, reference numeral 5 designates a throttle valve, reference numeral 6 designates an intake duct, reference numeral 7 designates an air cleaning, reference numeral 8 designates a crank angle sensor for detecting an engine speed, reference numeral 9 designates a vacuum introduction passage, reference numeral 10 designates an intake vacuum detector for detecting the pressure in the intake manifold 2 through the vacuum introduction passage 9, reference numeral 11 designates an EGR passage for connecting the exhaust manifold 3 to the intake manifold 2, reference numeral 13 designates an opening degree detector for an EGR control valve 12 with a pressure diaphragm arranged therein, reference numeral 18 designates an EGR control circuit, reference numeral 15 designates an atmospheric pressure introduction passage, and reference numeral 16 designates a control vacuum generator as a valve opening degree control device, which controls the opening degree of the EGR control valve 12 depending on an atmospheric pressure and a control vacuum to be controlled in accordance with an output signal from the EGR control circuit 18.
In the EGR control system for an internal combustion engine thus constructed, an engine speed N.sub.E and an intake vacuum P.sub.B which indicate the operating conditions of the engine are detected by the engine speed detector 8 and the intake vacuum detector 10, and N.sub.E and P.sub.B are inputted into the EGR control circuit 18. In the EGR control circuit 18 are set target opening degrees for the EGR control valve 12 which are determined depending on the operating conditions of the engine. The EGR control circuit forwards an output signal to the control vacuum generator 16 so that the comparison deviation between a determined target opening degree and an actual opening degree inputted through the opening degree detector 13 becomes zero. The control vacuum generator 16 adjusts an output vacuum by the intake vacuum based on the output signal forwarded from the EGR control circuit 18 to control the opening degree of the EGR control valve 12, determining the recirculation amount of the exhaust gas which can make the comparison deviation zero.
In other words, the opening degree of the EGR control valve 12 is feedback-controlled using the output from the opening degree detector 13 to obtain an EGR amount which fits to the operating conditions of the engine.
In such a conventional EGR control system, however, carbon particles or other particles included in the exhaust gas adhere to the EGR control valve 12 at a large amount because of long use of the valve 12 to introduce a change in the initially intended EGR amount corresponding to an opening degree of the EGR control valve 12, creating a problem in that it is impossible to carry out flow control with good accuracy. For example, if such a change causes an EGR ratio to increase, drivability would deteriorate, and if such a change causes the EGR ratio to decrease, the combustion temperature would rise to increase the amount of NOx components in the exhaust gas.
In addition, there is created a problem in that it is impossible to cope with an abnormality due to deterioration of a component part in the EGR control system.